Trimmer potentiometer with wiper interior of hollow wound resistance element



L. RESNICOW 3,537,057

ITH WIPER I ERIOR OF HOLLOW WOUND ISTANCE ELE NT 0&27, 1910 NT ME TRIMMER POTENTIOMETER W RES 2 Sheets-Sheet, 1 v

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A TTORNEY O Oct.27, 1970 I 3,537,051

TRIMMER POTENTIOMETER WITH WIPER INTERIOR 0F HOLLOW WQUND RESISTANCE ELEMENT Filed g 5 968 2 Sheets-Sheet 1 T v l :a 4

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' Arron/v5"? United States Patent TRIMMER POTENTIOMETER WITH WIPER INTERIOR 0F HOLLOW WOUND RESIST- ANCE ELEMENT Leon Resnicow, Stamford, Conn., assignor to Reon Resistor Corporation, Yonkers, NY. Filed Aug. 5, 1968, Ser. No. 750,718 Int. Cl. H01c /02 US. Cl. 338-180 17 Claims ABSTRACT OF THE DISCLOSURE A bobbin having two opposed slots supports a wound resistance wire the outer surface of which is encapsulated in plastic. Wiper and takeoif means that act oppositely in a direction substantially perpendicular to the longitudinal axis of the bobbin traverse the portions of the resistance wire exposed through the slots. The wiper means are part of a travelling nut assembly that is driven by a lead screw.

BACKGROUND OF THE INVENTION This invention relates to potentiometers and in particular to trimmer potentiometers.

Trimmer potentiometers are widely used in electronic circuitry to provide a convenient means of adjusting the circuit parameters. Such devices must be stable and must not be subject to change in resistance value after being set as a result of vibration, moisture, or other environmental conditions. In a device of this type it would be highly desirable to have the winding totally encapsulated. A problem in accomplishing this is the necessity of having a portion of the winding exposed for the purpose of making contact with a movable wiper.

One of the problems in producing encapsulated devices is the inherent danger of damaging fine resistance windings during manufacture. As will be explained more fully hereinafter, the design of the device is such that it may be manufactured by simple manufacturing techniques which do not require highly skilled technicians. The devices may be produced rapidly and etficiently with little loss in yield resulting from manufacturing problems.

SUMMARY OF THE INVENTION A resistance wire is wound on a slit bobbin. The outer portion of the Winding is encapsulated in a plastic body thereby maintaining the winding rigid. A wiper and takeoff are arranged to traverse the portion of the wire ex posed through the slot. Since the wire is encapsulated the wiper traversing the winding does not disturb the turns. A lead screw drives the wiper. The movable parts and exposed wiper portion are sealed oif from the environment.

There is described hereinafter a trimmer potentiometer construction and a unique method of making the potentiometer which permits of rapid inexpensive production.

It is an object of this invention to provide an improved trimmer potentiometer.

It is a different object of this invention to provide a method of forming an encapsulated trimmer potentiometer.

It is a different object of this invention to provide an improved brush assembly for a trimmer potentiometer.

It is a further object of this invention to provide a trimmer potentiometer requiring a minimum of assembly operations.

A still different object of this invention is to provide a trimmer potentiometer having a rigidly secured resistance winding.

3,537,057 Patented Oct. 27, 1970 Another object of the invention is to provide a potentiometer of the type described wherein the resistance element is sealed off from the environment.

A still diflFerent object of this invention is to provide a sealed resistance potentiometer not subject to change of setting under vibration.

Still a further object of this invention is to provide a high yield method of manufacturing of trimmer potentiometers of the type described.

These and other features, objects and advantages of the invention will, in part, be pointed out with particularity and will, in part, become obvious from the following more detailed description of the invention taken in conjunction with the accompanying drawing which forms an integral part thereof.

BRIEF DESCRIPTION OF THE DRAWING In the various figures of the drawing like reference characters designate like parts.

FIG. 1 is a view taken in elevation of a trimmer potentiometer of this invention;

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1;

FIG. 3 is an end view taken along line 3-3 of FIG. 2;

FIG. 4A is a plan view of a mandrel;

FIG, 4B is a plan view of a coil bobbin;

FIG. 4C is an end view of the coil bobbin of FIG. 4B;

FIG. 4D is a plan view of the coil bobbin with a winding formed thereon;

FIG. 4E is an end view of the form of FIG. 4D;

FIG. 4F is a plan view of a takeoff member and terminal;

FIG. 5 is an assembly of the components of FIGS. 4A-4F;

FIG. 6 is a pictorial view of the assembly of FIG. 5 coated with resin;

FIG. 7 shows the assembly of FIG. 6 positioned in a mold for encapsulation;

FIG. 8 is a pictorial view of the encapsulated assembly of FIG. 6 with the mandrel of FIG. 4A removed;

FIG. 9 is an exploded view showing the lead screw and wiper assembly in juxtaposition with the molded housing;

FIG. 10 is a pictorial view of a traveling nut wiper assembly;

FIG. 11 is a developed view of a traveling nut wiper employed in the present device; and

FIG. 12 is a side elevation of an alternative traveling nut wiper assembly.

FIG. 13 is a side elevation of an alternative traveling nut wiper assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 of the drawing there is shown an end view of the trimmer 10. Molded plastic body 12 has an exposed slotted head 16 of a lead screw 14. As may be seen in the sectional view of FIG. 2, the lead screw 20 is inserted into the plastic head by means of a shaft portion 18 which mates with bore 19 in an interference fit. The insulator head is used to insulate the circuit from the screwdriver used for adjustment, since, as will be explained later, a metal wiper assembly travels on metal screw 20. Preferably the screw is provided with a square thread. The wiper assembly can be insulated from the screw or the screw may be molded from a synthetic resin such as delrin, thereby eliminating the need for an insulator plastic head 16.

The head portion is provided with an annular groove 24 which carries an O-ring 26. The O-ring serves two primary functions: one, as a seal to isolate the interior of the body from the outer environment, and two, as

3 means to lock the screw against rotation. The lead screw is locked against axial movement by a pin 28 which looks in groove 30.

The resistance element consists of a resistance Wire 40 wound on an insulator plastic bobbin 35. The bobbin has an open slot 36 and a closed slot 38. The closed slot contains a takeoff wire 45. Terminal members 42 and 44 are attached to the ends of the windings 40 so as to be in electrical and physical connection therewith. Conventional welding, soldering, or brazing procedures are employed for this purpose.

A travelling nut and wiper assembly 31 rides on screw and carries interconnected wipers 32 and 34 which ride in slots 36 and 38, as shown in FIG. 3. Connection between a selected point on the winding and output terminal 46 is made to a voltage takeoff by means of the electrically interconnected wipers 32 and 34. The voltage takeofi shown in FIG. 4F consists of the fiat wire portion 45 and a rigid terminal portion 46 which is attached to member 45 as by welding, brazing, soldering, etc.

To produce the resistive elements, insulator plastic bobbin is slipped on mandrel 48 and placed in a winding machine and insulated resistance wire 40 wound over the bobbin. After winding, terminal members 42 and 44 are attached to the ends of the winding. The takeoff wire 45 is then laid into slot 38, as may be seen by reference to FIG. 4E.

The primary function of the disclosed mandrel arrangement is to insure accurate alignment between the lead screw and the winding. It will be noted that the ends of the mandrel form in the plastic body the bearing surfaces for the shaft. Pin 28 then locks the shaft against axial movement. While pin 28 is shown tangentially to the undercut of the shaft it would be practical to inseit the pin a radial direction. Another function of the mandrel is to prevent resin from becoming encrusted on the exposed face of the winding during the molding operation. Accordingly, it will be appreciated that the other forms of removable plugs, tape and protective coatings and barriers may be employed in place of the mandrel described heretofore.

The resistance wire coil is then bonded with an adhesive material. A preferred method is shown in FIG. 6 where the assembly, consisting of bobbin 35, resistance wire 40, terminals 42, 44 and 46, and voltage takeoff 45 mounted on mandrel 48, is coated with a resin 50. Epoxy resin is one example of a suitable coating which may be employed. After curing, the assembly is placed in a mold 52 which .is keyed to receive the mandrel and assembly in a predetermined position. The cavity 53 is then filled through sprue hole 55 with a second resin which does not affect the resin used for coating 50, the resulting molding being shown as member in FIG. 8. The mating cover of the mold is not shown as such molds are conventional.

The second resin may be a phenolic, by way of example.

Upon completion of the molding operation, the insulation is removed from the wire of winding 40 in the vicinity of slot 36 as by scraping it with a tool carrying an abrasive compound so as to expose the surface of the wire 41. Wiper 32 can now make contact to the winding. It is to' be noted that a feature of the method of manufacture is that the surface of the wire to be contacted is left protected until the very final stages of the assembly operation.

The lead screw head and traveling nut and wiper assembly are then inserted into the body and locking pin 28 (FIG. 2) inserted in bore 62 to lock the assembly in place. The bore '62 may be molded or drilled after molding. The bore 62 is preferably located outboard of the O- ring thereby permitting the O-ring to provide sealing against the environment. It will be appreciated that a completely sealed monolithic structure has been provided in which the resistance winding is rigidly maintained against movement and which is not subj ct 0 the effect of vibration.

The travelling nut assembly wiper assembly 31 shown in FIG. 10 carries a pair of spring wipers 32 and 34 which act in opposite directions that are substantially perpendicular to the longitudinal axis of the bobbin 18, the wipers 32 and 34 acting against the resistance wire 40 and take-off track 45. The balanced configuration is a highly desirable feature.

The travelling nut and wiper assembly may be formed from a single flat piece of beryllium copper or other suitable material, as shown in FIG. 11. The assembly is provided with ribs 22 which ride in the thread of the lead screw 20; thus the assembly also serves as a traveling nut. The assembly also includes guide members 73 which ride in the slots of the bobbin. The guides may be at right angles to the brushes as shown in FIG. 13.

The term longitudinal slot is intended to cover straight slots as well as helical slots which traverse the bobbin in a generally longitudinal direction.

Optionally, the lead screw 20 may be insulated from the wiper assembly by use of an insulator sleeve 61 as shown in FIG. 12. The sleeve 61 may be internally threaded to mate with the threads on the screw or in the alternative may be unthreaded and formed of a material such as Teflon or nylon and rely on frictional engagement for advancement. In the claims the term traveling nut is intended to encompass the latter and other like types of devices. The insulator 61 may be used to support separate wipers, which are electrically connected together.

The wipers per se may be formed of conventional electrically conductive brush materials such as graphite, precious metals and the like. The wiper-travelling nut assembly may be formed of conductive and non-conductive members. Since spring loaded wipers are employed, it is not essential that the take-off strip be perfectly parallel to the resistance element and therefore it should be understood that the term parallel as employed in the claims means substantially parallel.

It will be noted that the one-piece assembly has an open slot along the edge. This feature permits the cylinder to open under the pressure of the drive screw when the limit of axial travel is reached, thus providing the function of a slip clutch.

Optionally, bores 66 may be provided to act as mounting holes. Other conventional mounting means such as studs and brackets may be insert molded integrally with the case.

Without departing from the invention, the head and/or shaft may be made of insulating materials and may be formed of a monolithic piece. The molding process employed may be injection, transfer, compression, or other suitable processes. While it is preferred to employ insulated resistance wire, non-insulated wire may be used if adjacent turns are spaced apart.

There has been disclosed heretofore the best embodiment of the invention presently contemplated and it is to be understood that various changes and modifications may be made by those skilled in the art without departing from the spirit of the convention.

What I claim as new and desire to secure by Letters Patent is:

1. A variable resistor of the class described comprismg:

(a) a tubular walled sleeve of electrically nonconductive material, said sleeve having a longitudinal axis and a dimension along said axis substantially greater than the diameter thereof, said sleeve having a longitudinal slot extending radially through the wall;

(b) a resistance wire wound about the outside of said sleeve, whereby a portion of the winding is exposed to the cavity inside of the sleeve through the slot;

(c) a body of synthetic resin closely surrounding said resistance wire winding and having a longitudinal bore extending through the said body in communi\ cation with the interior of said sleeve;

(d) a lead screw extending longitudinally through the interior of said sleeve;

(e) a traveling nut riding on said lead screw;

(f) an electrically conductive wiper propelled by said traveling nut in contact with said exposed portion of said winding;

(g) a first terminal for connection to an external utilization circuit;

(h) conductive means interconnecting said wiper and said first terminal;

(i) at least one additional terminal, for connection to an external circuit, extending from said body and connected to said winding; and

(j) means carried by said body for limiting axial movement of said lead screw.

2. The apparatus of claim 1 wherein portions of said terminals are encapsulated in said body of snythetic resin.

3. The apparatus of claim 2 wherein said screw is provided with an annular groove and said means for limiting axial motion of said screw is a pin engaged in the groove.

4. The apparatus of claim 1 having a shaft portion extending through the bore.

5. The apparatus of claim 1 including a conductive strip mounted inside the sleeve parallel to the longitudinal axis of said sleeve.

'6. The apparatus of claim 1 including sealing means for sealing the interior of the sleeve from the environment surrounding the body.

7. The apparatus of claim 1 wherein said conductive means interconnecting said wiper and said first terminal includes a second wiper carried by said traveling nut and a conductive strip within said sleeve and parallel to the longitudinal axis, said second wiper being in contact with said conductive strip.

8. The apparatus of claim 7 wherein said first and said second wipers are diametrically opposed.

9. The apparatus of claim 1 wherein:

(a) said tubular sleeve includes an internal longitudinal slot extending radially outward from the inner surface;

(b) a conductive strip in the slot;

(c) a second wiper carried by said traveling nut, said second wiper being in contact with said conductive strip; and

(d) means electrically connecting said first named wiper and said second wiper, said last named means and said conductive strip comprising said conductive means interconnecting said wiper and said first terminal.

10. The apparatus of claim 1 wherein said lead screw terminates in a head portion disposed in the bore and means for sealing said head portion to said body whereby said lead screw may be rotated without breaking the seal.

11. The apparatus of claim 1 wherein said lead screw terminates in a head portion disposed in the bore wherein said head portion includes an annular groove having seated thereon an O-ring in compression between the walls of the groove and the inside of the bore.

12.. The apparatus of claim 1 wherein said winding is coated with a bonding agent for bonding together adjacent turns and said body of synthetic resin covers said bonding agent.

13. In a variable resistance device the combination comprising:

(a) an elongated resistance track;

(b) a conductive takeofli strip parallel to said track and spaced therefrom;

(c) a tube supporting said resistance track and said takeoff strip, said tube having longitudinal slots in the inner wall thereof, one of which contains said strip, and

(d) a wiper assembly having a first spring loaded wiper in contact with said resistance track, a second spring loaded wiper acting in contact with said takeoff strip and guide members engaging said slots in said tube; and

(e) drive means to move said wiper assembly along said resistance track.

14. The apparatus of claim 13 wherein said first spring loaded wiper is located on a side of said wiper assembly that is opposite that of said second spring loaded Wiper.

15. The apparatus of claim 13 wherein said wiper assembly is formed of a single sheet of metal.

16. The apparatus of claim 15 wherein said drive means comprises a lead screw and said wiper assembly includes bosses for engaging the thread of said screw.

17. The apparatus of claim 13 wherein said Wiper assembly is comprised of a tube having a generally longitudinal slit in the wall thereof whereby said tube will open when an outwardly acting radial force is applied.

References Cited UNITED STATES PATENTS 3,030,599 4/1962 Bourns 338-183 3,134,955 5/1964 Hardison 338- 3,151,308 9/1964 Root 338185 X 3,187,289 6/1965 Rolwes 338-180 3,233,201 2/1966 Layland 338-188 X LEWIS H. MYERS, Primary Examiner G. P. TOLIN, Assistant 'Examiner US. Cl. X.R. 338-148, 183 

